The present invention relates generally to an imaging system, and more particularly to an imaging system wherein toner images are successively formed on an image carrier using toners of two or more colors, and the images are then transferred onto an intermediate transfer medium at an applied transfer voltage, followed by transfer of the images on a recording material such as paper.
Among various imaging systems for forming color images known so far in the art, there is a specific imaging system wherein images successively visualized by toners of two or more are formed on a latent image carrier, an image with the colors put one upon another is formed on an intermediate transfer medium at an applied transfer voltage and then transferred onto a recording medium such as paper by one operation, and the toners are softened by the application of heat, pressure, etc. to fix toner images on the recording medium, thereby forming a color image.
To enhance the definition of the color image to be formed thereby reducing the amount of the toner used, it has been proposed to use toner particles having a reduced particle diameter.
However, a problem with the use of toner particles having such a reduced particle diameter is that frictional electrification of the toner particles with the surface of a development roller or a regulated blade becomes difficult, resulting in insufficient charges. Consequently, even a negative charge toner will unavoidably contain positively charged particles due to the presence of a charge quantity distribution in the toner, ending up with fog of a non-image area on an image carrier. To eliminate such fog, it is known to increase regulated pressure in a non-magnetic one-component development process. However, this can cause overcharging of toner, often resulting in a decrease in the toner density upon development or a transfer efficiency drop. To avoid these problems, JP06194943A proposes to control the amount of the toner deposited on a development roller in a proper range.
US2002076630 (JP2002131973A) proposes to use toner particles having a small diameter, thereby controlling the maximum amount of the toner to be deposited onto the recording material of each color in a given range and, hence, improving chargeability and particle image quality. However, this may be effective for improving the low-temperature fixation capability of the toner so that the toner is uniformly fixed, but is still insufficient for the transfer efficiency of the toner.
JP08248779A proposes a method for the formation of full-color images, wherein a latent image formed on a photosensitive member is developed with yellow, magenta and cyan toners as well as a black toner, each toner image is transferred onto an intermediate transfer medium, and an image developed with the black toner is superposed by primary transfer on the intermediate transfer medium and put by secondary transfer on other recording material.
The publication alleges that the intermediate transfer medium is not charged by repetition of the primary transfer so that the transfer efficiency of the black toner that is developed and primarily transferred in the last step is improved. However, the transfer efficiency of the toners is still less than satisfactory.
JP2000206755A proposes a color imaging system wherein for development a black toner is first used and yellow, magenta and cyan toners are then used, whereby mixing of the black toner with other color toners is so avoided that only the black toner can be recycled. However, the efficiency of transfer of the toners onto paper is again still insufficient.
JP200231933A proposes a color imaging system wherein toner images are formed on both sides of a recording material via an intermediate transfer medium, and yellow, magenta, cyan and black toner images are put one upon another in the order of cyan, yellow and magenta or vice versa, and black. However, the efficiency of transfer of the toners is still unsatisfactory.
JP10207164 proposes development of toners in ascending charge quantity order, and JP10260563 proposes to increase toner transfer voltage for each color, thereby enhancing transfer efficiency.
JP0527548A proposes to determine toner transfer voltage in such a way as to maximize the transfer efficiency of the lowermost toner layer, and JP200231933A proposes to use toners in the order of cyan, yellow and magenta or vice versa, and black.
For instance, JP05307310A teaches that development is carried out in the order of cyan, yellow, magenta, and black.
When toners of two or more colors are put one upon another for image formation, it is required to put the second and subsequent toners on the previously formed toner image; it is required that stable toner images be formed on the previously formed toner image.
Unless the second and subsequent toner images are precisely registered on the first toner image or at a position adjacent to the first toner image in the case of halftone, images having the desired color tone are hardly obtainable or image quality drops due to a scattering of toner particles.
When the formed toner images are transferred onto an intermediate transfer medium at a transfer voltage fed from a constant-voltage power supply, it is less likely to provide precise transfer of all the toner images or application of high transfer voltage is often needed.
One aspect of the present invention relates to an imaging system wherein an electrostatic latent image is formed on a latent image carrier, and a black toner or color toner of two or more colors are used to put colors one upon another so that the resultant image can be transferred and fixed onto an intermediate transfer medium or a recording material. According to this aspect, an object of the present invention is to take advantage of functional differences between the black toner and other color toners, thereby achieving a color imaging system, which enables a color image to be formed through a fixing step with high transfer efficiency but without causing misalignments of toner images obtained by transfer of toners onto an intermediate transfer medium or a recording material in a superposed fashion, and which enables the amount of the toners remaining on a photosensitive member upon transfer to be substantially reduced so that the quality of the resultant image can be improved.
Another aspect of the present invention also relates to an imaging system wherein toners of two or more colors are used on a photosensitive member to successively put colors one upon another on an intermediate transfer medium at an applied transfer voltage thereby forming a color image, which is then transferred by one operation onto a recording material such as paper or synthetic resin film, so that the color image can be fixed in a fixing step. According to this aspect, an object of the present invention is to provide an imaging system which enables a color image to be transferred with high transfer efficiency but without causing misalignments of the transferred color images, and which enables the amount of toners remaining on a photosensitive member upon transfer to be substantially reduced so that the quality of the resultant image can be improved.